Đáp án:
$\begin{array}{l}
A = \left( {\frac{1}{{\sqrt a - 1}} + \frac{1}{{\sqrt a + 1}}} \right):\left( {\frac{1}{{\sqrt a - 1}} - \frac{1}{{\sqrt a + 1}}} \right)\left( {a \ge 0;a \ne 1} \right)\\
= \frac{{\sqrt a + 1 + \sqrt a - 1}}{{\left( {\sqrt a - 1} \right)\left( {\sqrt a + 1} \right)}}:\frac{{\sqrt a + 1 - \left( {\sqrt a - 1} \right)}}{{\left( {\sqrt a - 1} \right)\left( {\sqrt a + 1} \right)}}\\
= \frac{{2\sqrt a }}{{\left( {\sqrt a - 1} \right)\left( {\sqrt a + 1} \right)}}.\frac{{\left( {\sqrt a - 1} \right)\left( {\sqrt a + 1} \right)}}{2}\\
= \sqrt a
\end{array}$
